Because small instances tend to die out in the long run. You can see this with OpenID. OpenID promised us, the users, a way to have one set of credentials to login everywhere. But years later you go to a new site and all you can set are just three options: Facebook, Google and Apple.

Choosing your instance which is not a top dog means that some day you'll have to migrate. So if you're smart enough you just sign up for the biggest instance from the start. And that will only speed up the decline of small instances.